Horizontal and Vertical X-mas Tree

A subsea Xmas tree is basically a stack of valves installed on a subsea wellhead to provide a controllable interface between the well and production facilities. It is composed of a variety of valves, which are used for testing, servicing, regulating, or choking the stream of produced oil, gas, and
liquids coming up from the well below. Different types of subsea Xmas trees may be used for either production or water/gas injection. Configurations of subsea Xmas trees may differ based on the
requirements of the projects and field developments. Functions of a subsea Xmas tree can be listed as
the following:

• Enable flow of the produced fluid from the well or the injection of water or gas from surface facility into the formation (called injection tree), including protection fluids, such as inhibitors for corrosion or hydrate prevention.
• Stop the flow of fluid produced or injected by means of valves in a safe way.
• Control the fluid flow through a choke (not always mandatory).
• Monitor well parameters at the level of the tree, such as well pressure, annulus pressure, temperature, sand detection, etc.

Basic xmas tree features

Source: http://asiaoec.com/wp-content/uploads/pdf/downloads/2010/John_Reid_CHARLES_TAYLOR_ENERGY.pdf

Subsea xmas tree - main development

• Integrated valve block
• Subsea chokes
• Chemical injection facilities
• Instrumentation
• Diverless ROV tooling and technology
• Deepwater
• High pressure / high temperature
• Metallurgy - CRA
• Protection
• Thermal insulation

There are two types of Xmas tree according to the configuration of valves, vertical Xmas tree and
Horizontal Xmas tree. Figure below shows the differences between two configurations.

Difference between horizontal Xmas tree configuration and vertical Xmas tree configuration

Source: http://brage.bibsys.no/xmlui/bitstream/handle/11250/183394/MPEMAS_2012%20Merve%20Dulger.pdf?sequence=1

 
Vertical Xmas Tree
The master valves are located above the tubing hanger and swab valves together with master valves are stacked vertically. The production and annulus bore lays vertically on the body of the tree. The well completion is finished before installing the vertical Xmas tree. Since the tubing hanger rests on the wellhead, Xmas tree can be recovered without having to recover the downhole completion. This type is generally applied in subsea fields due to their flexibility of installation and operation.

Vertical xmas tree

Source: https://www.geoilandgas.com/sites/geog.dev.local/files/styles/product_hero/public/570x495_MVXT.jpg?itok=vPTzO0wb

Horizontal Xmas Tree
In contrast to vertical Xmas tree, the valves of horizontal Xmas tree are located on the lateral sides of
the horizontal Xmas tree, allowing for easy well intervention and tubing recovery, thus this type of tree is very feasible for the wells that need many interventions. The tubing hanger is installed in the tree body instead of the wellhead. Consequently, the tree is installed onto the wellhead before completion of the well.

Horizontal xmas tree

Source: https://https://www.geoilandgas.com/sites/geog.dev.local/files/styles/product_hero/public/570x495_MHXT.jpg?itok=K6cA9O0p

VXT (Vertical Xmas Tree) / HVT (Horizontal Xmas Tree) technical comparison / differences 

• Configuration
• Installation equipment
• Installation sequence
• Repair sequence
• Capex - procurement / installation
• Opex - remedial action / repairs
• Project planning - operating philosophy

Typical subsea well construction sequence

Vertical xmas tree

1. Spud weel. Drill top hole
2. Run BOP stack on marine riser
3. Drill to TD. Run & cement liner
4. Run downhole completion & tbg hanger
5. Install temporary barriers
6. Recover BOP stack
7. Deploy and test xmas tree
8. Remove temporary barriers
9. Connect flowline jumpers and flying leads
10. Flow test well
11. Recover intervention package
12. Run xmas tree cap
13. Commission well from platform
14. Install protective cover

Horizontal xmas tree

1. Spud well. drill top hole
2. Deploy BOP stack on marine riser
3. Drill to TD. run and cement liner
4. Install temporary barriers
5. Recover BOP stack
6. Deploy and test xmas tree
7. Redeploy BOP stack on marine riser
8. Remove temporary barriers
9. Run downhole completion and tbg hanger
10. Test completion and interfaces
11. Flow test well
12. Install TH plug and internal tree cap
13. Connect flowline jumpers and flying leads
14. Recover intervention string
15. Recover BOP stack
16. Commission well from platform
17. Install protective cover

Cost comparison

• Purchase costs similar - varies according to spec and market conditions. Typically US$1-3mm
• Installation costs similar order of magnitude
• Replacement cost of HXT up to 10 times VXT. VXT can be replaced using drill rig or specially equipped DSV with lightweight intervention package. Typically US$5-10mm + mob/demob. HTX requires use of drill rig to run BOP stack and recover completion. Typically US$15-60mm + mob/demob.

Source:
http://brage.bibsys.no/xmlui/bitstream/handle/11250/183394/MPEMAS_2012%20Merve%20Dulger.pdf?sequence=1
http://asiaoec.com/wp-content/uploads/pdf/downloads/2010/John_Reid_CHARLES_TAYLOR_ENERGY.pdf